V.5 No 1

25

On physical processes in showering arcs

Just this typical appearance of showering arcs that is repeated in many studies, Mills took as the basis to explain the physics of processes which he stated in [2].

Fig. 2. Typical appearance of a section of showering arc which Mills took as the basis when described physical processes in showering arcs [2]

Fig. 3. Typical sequence of sections of voltage growth in contact gap which Mills took as the basis when described physical processes in showering arcs [2]

As we can see from Fig. 2 and 3 by Mills [2], in his model the process of voltage growth was shown as a linear section finished straight by the gap breakdown with voltage dropping down to that of arcing V_a .

True, not all diagrams had such shape. In Germer's work [3], general shape of two of 27 oscillograms some differed from shown in Fig. 1 (see Fig. 4 and 5) by a slight haze some blurring the background of upper part of oscillograms, but with that level of facilities, neither Germer nor other of listed researchers of that generation drew attention to this slight difference, and next generations simply never returned to this point and did not clear it.

Fig. 4. Typical mixed type of showering arcs that form an arc section between the initial and final stages of partition [3]

Fig. 5. One of diagrams that Germer yielded in studying the affection of cable length change on the pattern of transient voltage at the contacts; cable length here is 10 feet [3]

Thus, Mills theory is the basic for today, so it will be important for the below consideration to list briefly its main aspects. According to Mills, the showering arc production and caused by it contact surface erosion were characterised by the following sequence:

- the voltage growth at contacts after the bridge break relates to the charge of parasitic capacitance of contacts by the current of inductive load, up to the voltage at which the gap is broken;

- after the gap breakdown, the short arc, shown in Fig. 3 by horizontal sections between the sections of linear growth of voltage, arises between contacts even if the inductive current was insufficient to maintain it, as a large current of arc is provided by the parasitic capacitance of contact gap discharging through it;

- the voltage of opposite polarity arises in the oscillogram on account of voltage wave reflection from the opposite-sign inductive load; with it every time a new breakdown of gap takes place, just as in the alternating current discharge; Mills points that this causes the arc sections with inverse polarity, as shown in Fig. 3 between the second and third periods of showering arcs (while between the first and second periods the reverse polarity arc is absent);

- the short arc dies out, if the reflected wave voltage was insufficient for a repeated breakdown of the gap (under the reverse polarity);

- after the short arc has died out, the parasitic capacitance repeats charging;

- the cause of contact erosion under unstable discharges is the series of powerful short arcs.